1. Technical Field
The present invention relates to methods for isolating and purifying single-walled carbon nanotubes from contaminating materials, such as carbon and metal catalyst particles, present in the unpurified material following production of the single-walled carbon nanotube structures. Specifically, the present invention relates to utilizing solutions of suitable dispersal agents to isolate and purify individual single-walled carbon nanotube structures from a raw material including bundles of nanotube structures.
2. Description of the Related Art
There has been significant interest in the chemical and physical properties of carbon nanotube structures since their discovery in 1991, due to the vast number of potential uses of such structures, particularly in the field of nanotechnology, composite materials, electronics and biology. Accordingly, there has been an increase in demand in recent years for carbon nanotube structures for research and application purposes, resulting in a desire to produce in an efficient manner single-walled carbon nanotube (SWCNT) structures that are free of impurities and easily separable for their proper characterization.
The three most common manufacturing methods developed for the production of SWCNT structures are high pressure carbon monoxide (HipCO) processes, pulsed laser vaporization (PLV) processes and arc discharge (ARC) processes. Each of these processes produce SWCNT structures by depositing free carbon atoms onto a surface at high temperature and/or pressure in the presence of metal catalyst particles. The raw material formed by these processes includes SWCNT structures formed as bundles of tubes embedded in a matrix of contaminating material composed of amorphous carbon (i.e., graphene sheets of carbon atoms not forming SWCNT structures), metal catalyst particles, organic impurities and various fullerenes depending on the type of process utilized. The bundles of nanotubes that are formed by these manufacturing methods are extremely difficult to separate.
In order to fully characterize the physical and chemical properties of the SWCNT structures formed (e.g., nanotube length, chemical modification and surface adhesion), the contaminating matrix surrounding each structure must be removed and the bundles of tubes separated and dispersed such that each SWCNT structure may be individually analyzed. By maintaining an appropriate dispersal of individual SWCNT structures, characterization of the nanotubes formed may be accomplished in a mechanistic manner. For example, it is desirable to easily analyze and characterize dispersed SWCNT structures (e.g., determine change in nanotube length, tensile strength or incorporation of defined atoms into the carbon matrix of the SWCNT structure) based upon a modification to one or more elements of a manufacturing method.
It is further highly desirable to produce individual and discrete SWCNT structures in a form rendering the structures easily manipulable for use in the previously noted fields. At best, existing methodologies capable of physically manipulating discrete material components require elements that are measured on micron-level dimensions rather than the nanometer level dimensions of conventional partially dispersed and purified SWCNT structures. However, biological systems routinely manipulate with precise spatial orientation discrete elements (e.g., proteins) having physical dimensions on the order less than SWCNT structures. Thus, if SWCNT structures could be biologically derived so that biological “tools”, such as immunoglobulins or epitope-specific binding proteins, could be utilized to specifically recognize and physically manipulate the structures, the possibility of accurately spatially orienting of SWCNT structures becomes feasible. In order for this approach to be realized, the SWCNT structures must be individually separated from the raw material in a manner consistent with the optimal functioning of biological compounds during both the biological SWCNT derivitization and the manipulation processes. In other words, the SWCNT structures must be produced as individual, freely dispersed structures in an aqueous buffer system that exhibits a nearly neutral pH at ambient temperatures in order to effectively manipulate the structures.
Current methods for purifying and isolating SWCNT structures by removing the contaminating matrix surrounding the tubes employ a variety of physical and chemical treatments. These treatments include high temperature acid reflux of raw material in an attempt to chemically degrade contaminating metal catalyst particles and amorphous carbon, the use of magnetic separation techniques to remove metal particles, the use of differential centrifugation for separating the SWCNT structures from the contaminating material, the use of physical sizing meshes (i.e., size exclusion columns) to remove contaminating material from the SWCNT structures and physical disruption of the raw material utilizing sonication. Additionally, techniques have been developed to partially disperse SWCNT structures in organic solvents in an attempt to purify and isolate the structures.
All of the currently available methods are limited for a number of reasons. Initially, it is noted that current purification methods provide a poor yield of purified SWCNT structures from raw material. A final SWCNT product obtained from any of these methods will also typically contain significant amounts of contaminating matrix material, with the purified SWCNT structures obtained existing as ropes or bundles of nanotubes thereby making it difficult to analyze and characterize the final SWCNT structures that are obtained. These methods further typically yield purified SWCNT structures of relatively short lengths (e.g., 150-250 nm) due to the prolonged chemical or physical processing required which causes damage to the nanotubes. Additionally, a number of isolation techniques currently utilized require organic solvents or other noxious compounds which create environmental conditions unsuitable for biological derivitization of SWCNT structures. Organic solvents currently utilized are capable of solubilizing SWCNT structures in bundles and not individual, discrete tubes. Furthermore, present isolation techniques require prolonged periods of ultra-speed centrifugation (i.e., above 100,000×g) in order to harvest nanotube structures from solvents or other noxious compounds used to remove contaminating matrix material from the nanotubes.
Presently, the overwhelming problem for industrial and academic laboratories engaged in the use of carbon nanotubes for research as well as other applications is the limited source of discrete, completely separated SWCNT structures. Investigations into the vast potential of uses for SWCNT structures are being hampered by the limited supply of well characterized SWCNT material free of significant amounts of contaminants like amorphous carbon and metal catalyst particles.
Accordingly, there presently exists a need for harvesting high yields of purified SWCNT structures from the raw material of a carbon nanotube production process in a fast and efficient manner to meet the demand for such structures. Additionally, it is desirable to provide SWCNT structures as discrete and individual structures (i.e., not bundled together), having suitable lengths and well characterized for biological derivitization and easy manipulation.